The invention relates to filter media and filter assemblies for filtering fluid including air.
The invention arose during development efforts directed toward increasing filter life in filters exposed to high dust concentrations of dry and/or sooty and/or oily particles. Prior non-woven fibrous filter media use needling techniques to achieve required media thickness and solidity. During the process of needling, small holes are formed through the media. Such needle holes, formed in felted and other synthetic filter materials, promote the penetration of incoming and detached particles. These needle holes cause discontinuities in the filter media structure because they are relatively large pores compared to those in the bulk fiber matrix. The large pores offer a path of low resistance to the fluid flow. Because smaller pores are clogged faster by deposited particles, the velocity through the larger needle holes increases correspondingly. The holes caused by needle punching and the increased velocity therethrough thus create conditions for particle penetration and reentrainment. Consequently, both uncaptured and detached dust particles can penetrate the filter through such needle holes, leading to lower filter efficiency.
Further in the noted prior fibrous filter media, the fibers extend dominantly perpendicular to the flow direction of fluid through the filter media. In this type of orientation, the surface area of particle contact and adhesion with the fibers is small while the face surface area of particle clusters is large as captured dust particles build on each other. Sometimes dust particle aggregates form bridges between fibers. Such particle aggregates and bridges can be easily blown off, particularly if exposed to high fluid flow rates or pulsation. The particles form aggregates which can form particle bridges between fibers that block flow through the media and shorten filter life due to premature clogging. Furthermore, such particle aggregates and bridges are subject to instability and the noted break-off, particularly in areas adjacent the noted needle holes through which the air velocity increases. Such needle holes may be penetrated by an increased number of particles, which is undesirable because of such release and reentrainment of contaminant particles.
Contaminant cake stability is another problem in prior fibrous non-woven filter media. Because such media has very little rigidity and compression resistance, the contaminant cake which builds on the filter media can be easily dislodged in localized areas. The noted prior needling of such media improves rigidity and compression resistance when compared to air-laid nonwovens, however there is a need for additional stability of the contaminant dust cake to improve filtration performance. Furthermore, the noted needle holes cause the noted large pores which cause a nonuniform distribution of the dust cake, i.e. large craters at the pores, and dust cake particle agglomeration and bridging between fibers at areas between such needle hole pores.
The present invention provides a simple and effective solution to the above noted and other filter problems. In one desirable aspect, the invention eliminates needling and the needle holes caused thereby in fibrous filter media, including non-woven media. This eliminates the noted large pores in the bulk fiber matrix at needle holes, which in turn eliminates the noted path of low restriction to the fluid flow and increased velocity therethrough, thus eliminating the noted penetration of uncaptured and detached reentrained dust particles through such needle holes, otherwise causing lower filter efficiency.
In another aspect, the surface area of particle adhesion and contact to fibers is increased and the face area of dust particle aggregates and bridging between fibers is reduced, preventing blockage to flow otherwise caused thereby, and reducing risk of reentrainment, and providing a more stable filtration process.
In another aspect, rigidity and compression resistance of the filter media is enhanced, increasing contaminant cake stability. In addition, the invention enables a more uniform distribution of a stable dust cake, including elimination of the noted craters otherwise formed at needle holes, and contaminant particle aggregates and bridging between fibers at areas between such craters, which in combination with the noted increased rigidity and compression resistance of the filter media, enhancing contaminant cake stability.
The present invention provides an improved method of filtering particles in fluid flowing along a given flow direction through filter media having a plurality of fibers, including an improved method for increasing capture of and retention of contaminant particles by the fibers of the filter media. In one aspect, the method includes, in combination: increasing residence dwell time of particles moving along the fibers to increase the chance of, and extend the time of, fiber-particle contact; increasing Brownian diffusion probability of particles diffusing to the fibers; increasing the spread of, and even loading of, particles along the fibers; reducing particle agglomeration and particle bridging between fibers which would otherwise block flow and be subject to instability and reentrainment if exposed to high flow rates or pulsation. The increased residence dwell time, increased Brownian diffusion, increased spread and evenness of particle loading, and reduction of particle agglomeration and bridging, is accomplished by orienting the fibers parallel to the fluid flow direction in combination with providing fibers of sufficient length along the flow direction such that residence dwell time is increased, Brownian diffusion is increased, spread and evenness of particle loading is increased, and particle agglomeration and bridging is reduced, all as compared to, and relative to, filter media fibers extending perpendicular to the fluid flow direction.
The contact surface area between the contaminant particles and their aggregates remains large, and the face area of the contaminant aggregates stays small during contaminant loading. This is a favorable condition for a stable filtration process.
A significant advantage of the invention is higher contaminant capacity due to favorable contaminant loading. In the noted prior art, contaminant accumulates on the fibers and aggregates on itself and also forms long and fragile bridges between fibers blocking fluid flow. In the present invention, contaminant particles forming the contaminant cake are distributed more evenly and uniformly along the entire fiber length. Contaminant loading is thus more evenly and uniformly distributed not only along a surface area along a plane perpendicular to the flow direction but also along a fiber length extending along a plane parallel to the flow direction. This uniformity of contaminant distribution along both planes provides decreased filter pressure drop and increased contaminant loading capability.
In a further desirable aspect, the invention increases rigidity and compression resistance of the fibrous filter media, including at low solidity, and enhances stability of the uniformly distributed contaminant cake, including resistance to reentrainment even if exposed to high flow rates or pulsation.
In preferred form, filter media construction is provided including a sheet of filter material having a thickness dimension between oppositely facing first and second sides for filtering particles in fluid flow therethrough along a flow direction perpendicular to the sheet and the first and second sides and parallel to the thickness dimension. The sheet is formed by an internal pleated subsheet of a plurality of fibers and having a plurality of pleats extending between first and second sets of pleat tips, the pleats extending parallel to the flow direction, the first set of pleat tips providing the first side of the sheet, and the second set of pleat tips providing the second side of the sheet. The pleats engage each other and are packed sufficiently tightly such that fluid flows along the flow direction through the pleats in parallel therewith, rather than between the pleats and then transversely therethrough. The subsheet has a pre-pleated planar condition with the fibers extending dominantly parallel thereto and dominantly unidirectionally parallel to each other. The subsheet has a pleated condition forming the sheet, wherein the fibers extend along the pleats substantially parallel to the flow direction, and the fibers extend around the pleat tips substantially parallel to each respective side of the sheet and substantially perpendicular to the flow direction. The pleats are preferably bonded to each other to prevent flow of fluid therebetween along the flow direction parallel to the sheets. The bonding of the pleats and fibers to each other also enhances rigidity and compression resistance of the sheet along the thickness dimension. This eliminates the above noted needle holes associated with needling. The enhanced rigidity and compression resistance of the sheet along the thickness dimension also increases contaminant cake stability. The sheet may be used as a high capacity filter or as a prefilter, to increase total filter efficiently and capacity.
In a further aspect of the invention, the noted filter media construction enables the noted subsheet to be non-needled and non-woven and to have sets of different fibers therein, including different diameter fibers and fibers with various surface charges including opposite polarity fibers. The latter are desirable to enable use of the known triboelectric effect. Such triboelectric effect is particularly desirable in the presently enabled construction because the oppositely charged or differentially charged fibers define a plurality of electric field flux lines therebetween, which flux lines extend dominantly perpendicular to the flow direction and are stacked along the flow direction in a plane parallel thereto, such that fluid flows along such plane and perpendicular to the flux lines and cuts serially sequentially across plural flux lines, increasing the chances of triboelectric capture due to the increased number of flux lines crossed. The triboelectric effect is particularly useful for capturing small particles which can clog a downstream main or primary filter. In a further aspect, the present filter media construction enables the use of differing diameter fibers to enhance efficiency.
In another aspect of the invention, an extended life fluid filter assembly is provided, including first and second filter elements. Each filter element has pleated filter media with a plurality of pleats extending between first and second sets of pleat tips. The pleats of each filter element are substantially parallel to the fluid flow direction and extend along such flow direction between respective pleat tips. The pleats of one of the filter elements are spaced from each other along a spacing direction transverse to the flow direction and define a transverse gap therebetween through which fluid flows along the noted flow direction. Such fluid then turns and flows transversely through the pleats of such filter element. The pleats of the other filter element are packed against each other and bonded together without a transverse gap and block fluid flow therebetween. The fluid flows along the flow direction through such pleats of such other filter element in parallel therewith, rather than transversely therethrough.
In a preferred embodiment, the extended life fluid filter assembly includes a main filter element and a prefilter element. The main filter element is provided by pleated filter media having a plurality of pleats extending between first and second sets of pleat tips. The pleats extend generally parallel to the fluid flow direction and are spaced from each other along a spacing direction transverse to such flow direction and defining a transverse gap therebetween through which fluid flows, such that fluid flows along the flow direction between the pleat tips of the first set and then transversely through the pleats and then along the flow direction between the pleat tips of the second set. The prefilter element is adjacent and upstream of the main filter element, may or may not be bonded to it and is provided by a sheet of filter material having a thickness dimension between oppositely facing first and second sides for filtering particles in fluid flow therethrough along the flow direction perpendicular to the sheet and the first and second sides and parallel to the thickness dimension. The second side is adjacent the main filter element. The sheet is formed by an internal pleated subsheet composed of a plurality of fibers and having a plurality of pleats extending between first and second sets of pleat tips. The pleats of the subsheet extend parallel to the fluid flow direction. The first set of pleat tips of the subsheet provide the first side of the sheet. The second set of pleat tips of the subsheet provide the second side of the sheet. The pleats of the subsheet engage each other and are packed sufficiently tightly against each other such that fluid flows along the flow direction through the pleats of the subsheet in parallel with such pleats, rather than between such pleats and then transversely therethrough. The subsheet has a pre-pleated planar condition with the fibers extending dominantly parallel thereto and dominantly unidirectionally parallel to each other. The subsheet has a pleated condition forming the sheet, wherein the fibers extend along the pleats of the subsheet substantially parallel to the flow direction, and the fibers extend around the pleat tips of the subsheet substantially parallel to each respective side of the sheet and substantially perpendicular to the flow direction.
In a further aspect in combination with the noted two stage fluid filter assembly, the invention provides a desirable filtering method increasing uniformity of the contaminant cake on the prefilter by reducing particle agglomeration and particle bridging between prefilter media fibers which would otherwise block flow and reduce uniformity and be subject to instability and reentrainment. This is provided by orienting the fibers dominantly parallel to the flow direction to more evenly load contaminant particles along the entire length of the fibers as compared to, and relative to, filter media fibers extending perpendicular to the flow direction. This provides uniformity of the contaminant cake on said prefilter along a plane parallel to the flow direction. Uniformity of the contaminant cake is also provided along a plane perpendicular to the flow direction by eliminating the above noted craters at needle holes.